Nineteenth Annual Meeting. 



119 



ments as indigo and ultramarine. That the world should be flooded with light of 

 the sky's deepest blue, and that nevertheless we should be spared even a trace of the 

 very peculiar effects produced when 

 we substitute blue glass in the win- 

 dows of a room for that of the 

 usual neutral tint, or color the 

 walls with ultramarine, is, to say 

 the least, remarkable. One might 

 suppose that whenever light from 

 the sky, undiluted by the direct 

 rays of the sun, became the chief 

 source of illumination, its influ- 

 ence would be obvious to the most 

 careless observer; modifying the 

 colors of every object, and pro- 

 ducing a thousand marked changes 

 in the aspects of nature. The re- 

 verse is true, and even the multi- 

 tude of less obvious phenomena 

 with which only the artist and the 

 special student of color might ex- 

 pect to meet with in consequence 

 of the blueness of light from the 

 sky are lacking. In clear weather 

 the daylight which penetrates every 

 nook and cranny of the inhabited 

 world is sky light, save where the 

 direct rays of the sun may chance 

 to fall, and it is found to vary 

 from direct sunlight in no respect, 

 excepting in intensity. 



We are not in the habit of as- 



r Hi. t. 

 signing to it even that degree of 



blueness which in the popular mind is associated with moonlight. Even those who 

 use the spectroscope have long since noticed the absence of any marked difference 

 between light from the sun, that reflected from white clouds, and that which reaches 

 us from the open skj'.^ 



In the spectrophotometer we have a modification of the spectroscope by means 

 of which it is possible to compare the intensity of spectra, wave-length for wave- 

 length. An analysis of colors with this instrument shows all of them, excepting the 

 tints of the pure spectrum, to be modifications of white in which one portion or an- 

 other has been destroyed by absorption within the substance to which the color be- 

 longs. Upon the screen you see the spectrophotometric curves characteristic of four 

 well-known pigments, viz.: red lead, chrome yellow, chrome green, and artificial 

 ultramarine, (Fig. 4.)2 Such curves, indicating as they do the relative intensities 



1 In exception to this statement stand, however, observations by Rayleigh, ( Philosophical Magazine, 

 1871,) who found in the spectrum of the open sky a sligU excess of the more refrangible wave-lengths, 

 and more recently by H. C. Vogel, (Photographische Mittheilungen 20, 1883, p. 74,) who found evidence 

 in photographed spectra of the sun and of the sky, of a preponderance of ultra violet rays in the 

 latter. 



- Taken from the author's paper entitled, "A Spectrophotometric Study of Pigments," Am. Journal 

 of Science, vol. 28, 1884. 



